The invention relates to a method of stereophonic recording wherein sound waves are detected by head-mounted stereophonic microphones in a pair of actual or simulated human ears, and the electrical signals at the microphone outputs are recorded, e.g., on a stereophonic tape recorder.
In the technique of "artificial head" stereophony, a head formed from plastic or other synthetic material and having ears whose acoustic characteristic simulate that of a particular human model is employed at a pick-up location, e.g., in a concert hall. The resulting sound pressure produced in the ear canals of the artificial head is recorded via a pair of correspondingly located stereophonic microphones.
This technique has been found to be subject to inaccuracies when a listener hearing the resulting record attempts to orient himself with respect to the original source of the sound, particularly within certain angular ranges. More important, since the acoustic frequency response of a human head to sound waves includes a long-term storage and correction aspect supplied by the brain by which each person receives sound impressions in a particular way, stereophonic recording employing an artificial head cannot approach the realism of the original sound source to the listener. This is particularly important in the field of amateur stereophonic recording where the reproduction is primarily desired for the subjective enjoyment of the original person doing the recording.
Such lack of realism is augmented by the fact that although the reference location in normal recording microphones is within the ear canal itself, the ideal reference position for known types of reproduction apparatus, particularly earphones, is slightly outside the ear canal. This relative displacement in reference points leads to distortion during the reproduction of the stereophonic recording by a human listener wearing head-mounted earphones in the usual manner. One reason for such distortion is the unavoidable shaping of the frequency characteristics of the sound waves by the impedance of the ear canal during the recording operation, and is aggravated by the use of elongated sound guides commonly used in known recording apparatus.
Additionally, since the acoustic characteristics of human ears are extremely varied, a recorded pick-up by an artificial head designed to match a particular human model will generally lead to distortion when the recording is played back in the ears of humans having dissimilar acoustic characteristics.